Presently, group III-nitride-based transistors such as gallium nitride (GaN)-based high electron mobility transistors (HEMTs) are typically depletion-mode (D-mode) devices. D-mode devices are on (e.g., conduct current in the channel) at a zero voltage of the gate with respect to the source, and must be pulled down to a negative pinch-off voltage (also referred to as threshold voltage) to pinch off the current flow. In contrast, enhancement-mode (E-mode) devices, which are off at a zero gate voltage and use a positive gate voltage to turn on the E-mode device, may be desirable for applications such as power switching. However, conventional GaN HEMT structures with termination layers directly in contact with the Schottky metal stack exhibit significant gate leakage which can vary over time due to thermal or electrical stresses. These structures are unsuitable for E-mode devices.